Process for controlling the brightness of paper products

ABSTRACT

A process for shading white paper characterized in that a mixture comprising at least one direct dye or pigment and at least one optical brightener is used, is claimed as well as the mixture and the paper dyed with this method.

[0001] This invention relates to a process for controlling the Brightness of paper products while using shading dyes.

[0002] In the paper industry Brightness is the technical term used to describe the reflectance of the paper or pulp at a specific wavelength, e.g. TAPPI Brightness, ISO Brightness etc.

[0003] There are several definitions for Brightness, Whiteness and color of paper samples.

[0004] a) CIE Whiteness and CIE Tint

[0005] CIE Whiteness and CIE Tint are calculated from the chromaticity co-ordinates and the color coordinates. CIE Whiteness gives an impression of the whiteness of a sample including OBA and shading dye elements. The tint value is used to describe the particular shade of white. This gives fairly close correlation with the human eye and undoubtedly the best method of measuring whiteness available. In addition the Tint value is useful since two whites can have the same whiteness but have different shades. CIE Whiteness and Tint are defined in ISO 11475 as: CIE Whiteness W₁₀ = Y₁₀ + 800(x_(n · 10) − x₁₀) + 1700(y_(n · 10) − y₁₀)

[0006] CIE Tint T = 900(x_(n · 10) − x₁₀) − 650(y_(n · 10) − y₁₀)

[0007] b) CIELAB L*, a*, b* Values

[0008] The CIELAB L*, a*, b* values are used to describe color space, they are the three axis of a three dimensional system of describing a color. L* represents the difference between black and white, a* is the red-green co-ordinate and b* is the blue yellow co-ordinate. They are particularly useful for describing and controlling the effect of shading dyes in white papers. Another similar system is L, C, H where C and H represent the Chroma and the Hue. Whilst the a* and b* value are very useful for describing and controlling shade, the L* value again does not take into account shading dyes. When shading dyes are added the L* value is decreased. It should be noted also that OBA's effect a* and particularly b* values. CIELAB L*, a*, b* values L* value: Lightness (White-Black co-ordinate) 0 = perfect black 100 = perfect white a* value Red-green co-ordinate b* value Yellow-blue co-ordinate

[0009] Note that in today's paper industry, shading dyes are not only added to achieve a whitening effect, but they are also added to control the color. Control of the color is often achieved through the use of a* and b* target values. This can be done manually, on-line or by on-line closed-loop color control. Violet and reddish blue dyes are used to improve whiteness, other colors are used to control the shade of the white paper, for example, reds, blues and yellows. Black colors are also used in very small amounts to control the opacity of white papers.

[0010] c) Brightness

[0011] Brightness is a term used in the paper and pulp industry, which refers to a reflectance factor, usually around 460 nm. It is defined in ISO 2470 and 3688, though there are other definitions (e.g. Scandinavian Pulp, Paper & Board testing committee SCAN-test C11:75 & TAPPI standard test method T452). When small amounts of shading dye are added to white papers they reduce Brightness to a greater or lesser extent depending on the dye and concentration used. This occurs because dyes absorb light and therefore reduce reflectance. Optical Brighteners increase Brightness as they are fluorescent and therefore increase the Brightness value.

[0012] Optical brighteners are colorless to weakly colored organic compounds that, in solution or applied to a substrate, absorb ultraviolet light (e.g., from daylight at ca. 300-430 nm) and re-emit most of the absorbed energy as blue fluorescent light between ca. 400 and 500 nm.

[0013] In daylight optical brighteners can thus compensate for the aesthetically undesirable yellowish cast found in white industrial substrates, such as textiles, papers, or plastics. Furthermore, since a portion of the daylight spectrum not perceived by the eye is converted to visible light, the brightness of the material is enhanced to give a dazzling white.

[0014] The first method of compensating for the yellowish cast was the use of a blue dye such as ultramarine or indigo. Bluing lowers the reflectance of the sample in the long-wavelength part of the visible spectrum. As a result, the sample takes on a neutral white appearance, which appears more appealing to the eye.

[0015] Optical brighteners lower the reflectance mainly in the UV and near visible by absorption; at visible wavelengths (mostly with a maximum at 435-440 nm), they greatly increase the Brightness through fluorescence. The optical brightener acts as a supplementary emission source. Optical brighteners are more effective the cleaner and whiter the substrate is.

[0016] In a typical white paper production process many color parameters are controlled, those in use include:

[0017] CIE Whiteness

[0018] ISO Brightness

[0019] CIELAB a* value

[0020] CIELAB b* value

[0021] CIELAB L* value

[0022] Fluorescence (usually obtained by measuring Brightness or Whiteness with UV component and subtracting the value measured without the UV component in the illuminant)

[0023] The problem for the paper maker is that the two types of components used to control these parameters are: a) one or more shading dyes b) one or more OBA's

[0024] (Shading dyes are typically added at the wet-end or in coatings and rarely in the size-press. OBA's are added in the wet-end, size-press and or coating.) These components have an effect on each of the measured values, sometimes the desired addition rate required to adjust a required parameter has a negative influence on another parameter which must then be corrected, for example:

[0025] Reddish Violet Shading dye (e.g. C.I. Direct Violet 51, C.I. Direct Blue 267) or pigment (e.g. C.I. Pigment Violet 23): Used to adjust a* and b* values, has a negative effect on Brightness and a positive effect on Whiteness, no effect on fluorescence.

[0026] Bluish/Neutral Violet Shading dye (e.g. C.I. Direct Violet 35) or pigment (e.g. C.I. Pigment Blue 15, C.I. Pigment Violet 3)): Used to adjust b* value, has a negative effect on Brightness and a positive effect on Whiteness, little or no effect on a* value, no effect on fluorescence.

[0027] Red Shading dye (e.g. C.I. Direct Red 253): Used to adjust a* value, has a negative effect on Brightness and no effect on Whiteness, little or no effect on b* value,, no effect on fluorescence.

[0028] OBA (e.g. C.I. Fluorescent Brightener 220, C.I. Fluorescent Brightener 28, C.I. Fluorescent Brightener 264): Used to increase Fluorescence and Whiteness and Brightness, also influences a* and b* values.

[0029] In the case of the production of a high white paper involving a violet, red or blue shading dye and an OBA added at the wet-end and possibly another OBA added at the size-press, the decision making process to adjust the additions to get the right target parameters for process control can be seen to be very complicated, especially when an increase or a decrease in the shading dyes or OBA's must be considered. When any shading dye dosage is increased the Brightness goes down, therefore the paper maker must compensate by adding more OBA to regain the Brightness target, vice-verca when the shading dye dosage is decreased. The components must often be adjusted together to achieve all of the target parameters.

[0030] The problem of the decrease of the Brightness* while using shading dyes is a widely known problem.

[0031] In the state of the art this problem was solved by adding additional optical brighteners. Until now a typical procedure for doing it was:

[0032] adding first the solution containing the dyestuff and afterwards adding the solution containing optical brighteners.

[0033] The color control in a white paper dyeing process is difficult to obtain. When increasing the concentration of the shading dye to achieve the correct color, the concentration of the OBA then has to be increased to obtain the right Brightness, since the increase in shading dye addition causes a decrease in Brightness. This complicated and delicate procedure has to be done either manually or by closed loop color control.

[0034] The goal of the present invention is to provide an easier method to control the Brightness in paper products while using shading dyes by using a one step procedure which will help to save not only time but also reduce the necessary amount of the dyestuff and the OBA.

[0035] A process for shading white paper is provided, characterized in, that a mixture comprising at least one direct dye or pigment and at least one optical brightener is used.

[0036] Preferably the inventive process is characterized in that the mixture consists of one or more direct dyes and one or more optical brighteners in a ratio of 25/75 to 90/10 percent by weight, preferably 50/50 percent by weight.

[0037] Also a part of this invention is a mixture for adding fluorescence to shading dyes for paper comprising one or more direct dyes and one or more optical brighteners in a ratio of 25/75 to 90/10 percent per weight.

[0038] The OBA's for the present invention are Wet-end OBA's, Size-press OBA's and/or Coating OBA's.

[0039] Preferred OBA's are C.I. Fluorescent Brightener 220, C.I. Fluorescent Brightener 28 and/or C.I. Fluorescent Brightener 264.

[0040] Paper obtained by the inventive process is also a part of this invention.

[0041] With this method the shading dye dosage can now be adjusted without affecting the Brightness value and allowing only one adjustment to be made, to the shading dye, without needing to also adjust the optical brightening agent dosage to regain Brightness.

[0042] For this invention all known OBA's for paper can be used. Preferred optical brighteners are Wet-end OBA's (Leucophor® AP, AL, U etc.), Size-press OBA's

[0043] (Leucophor® U, NS, SHR, TC etc) Coating OBA's (Leucophor® U, AP, AL, NS, SHR, TC etc.). Leucophor is a registered trademark of Clariant.

[0044] The OBA is added to the shading dye in the form of a mixture so that, when added to the paper the fluorescence of the OBA compensates for the loss in Brightness due to the absorbance of the dye and the Brightness of the paper is not reduced by the addition of the shading dye. Therefore process control is made more simple as only one component needs to be adjusted to achieve a color target and the Brightness is not reduced, therefore a compensatory adjustment of the OBA is not needed.

[0045] An additional benefit is that the amount of the shading dye used is reduced since the mixture of OBA and shading dye has a greater effect on the color co-ordinates (a*,b*) than the dye alone.

[0046] Pigments (e.g. C.I. Pigment Violet 23, C.I. Pigment Blue 15, C.I. Pigment Violet 3) are also used in the shading of white papers (for example in coatings).

[0047] Shading dyes and OBA's are also used in size-press and coating applications as well as wet-end applications.

[0048] The following example illustrates the present invention.

EXAMPLE 1

[0049] A standard preparation of paper stock consisting of 50% bleached softwood pulp and 50% bleached pulp are prepared at 20° Schopper Riegler, 20% CaCO₃ are added (ash by weight of dry fiber). Handsheets are prepared from the above stock containing various amounts of a 1:1 mixture of a liquid anionic direct paper dye (containing 13% C.I. Direct Violet 35) and a mixture of a liquid optical brightening agent (containing 25% C.I.F. Br. 290) and, as a control a 50% dilution of the same liquid paper dye (C.I. direct Violet 35). A sizing agent is added (1.7% Aquapel® 360X) and a retention aid is used to retain the chalk filler (0.003% Cartaretin® 20CE). The following additions are made of the dye and the dye/optical brightening agent-mixture, by weight of liquid product on dry fiber:

[0050] 0.0%, 0.001%, 0.003%, 0.006% and 0.100%.

[0051] Afterwards the hand-sheets are pressed and dried. The following table shows the results of the measurements, made on a Minolta CM-3700d Spectrometer, with the UV component included. CIE Sample Whiteness Brightness a* b* 0.0% (Blank) 71.7 85.7 0.1 4.4 0.001% (Dye only) 74.0 86.0 0.0 3.7 0.001% (50% dye/50% Optical 74.3 86.0 0.0 3.6 brightener agent) 0.003% (Dye only) 76.4 85.8 0.1 2.8 0.003% (50% dye/50% Optical 77.4 86.0 0.1 2.6 brightener agent) 0.006% (Dye only) 79.4 85.4 0.1 1.7 0.006% (50% dye/50% Optical 83.1 86.3 0.2 0.9 brightener agent) 0.100% (Dye only) 114.9 79.4 2.2 −9.7 0.100% (50% dye/50% Optical 134.1 84.5 3.6 −13.7 brightener agent) 

1. A process for shading white paper by using a mixture comprising at least one direct dye or pigment and at least one optical brightener characterised in that the mixture consists of one or more direct dyes and one or more optical brighteners in a ratio of 25/75 to 90/10 percent by weight, preferably 50/50 percent by weight.
 2. A process according to claim 1 characterized in that C.I. Direct Violet 51, C.I. Direct Blue 267, C.I. Direct Violet 35 and/or C.I. Direct Red 253 are used.
 3. A process according to claim 1 characterized in that C.I. Pigment Violet 23, C.I. Pigment Violet 3 and/or C.I. Pigment Blue 15 are used.
 4. Mixture for adding fluorescence to shading dyes for paper comprising one or more direct dyes and one or more optical brighteners in a ratio of 25/75 to 90/10 percent per weight.
 5. Mixture for adding fluorescence to shading dyes for paper comprising one or more pigments and one or more optical brighteners in a ratio of 25/75 to 90/10 percent per weight.
 6. A process according to any one of claims 1 to 3 or a composition according to claim 4 or 5, characterized in that the optical brightener are Wet-end OBA's, Size-press OBA's and/or Coating OBA's.
 7. A process according to claim 6, characterized in that the optical brightener C.I. Fluorescent Brightener 220, C.I. Fluorescent Brightener 28 and/or C.I. Fluorescent Brightener
 264. 10. Paper obtained by the process of claim 1 or 2 or 3 or 6 or
 7. 